Presently existing liquid level detectors having conductivity probe sensors have not always operated with satisfactory effectiveness and occasionally have provided false readings. In a typical installation, a probe is suspended at the end of an electric conductor which is connected to a sensor. Another conductor connects the sensor to a metal tank filled with a conductive liquid. When the probe comes in contact with the conductive liquid, an electric circuit is completed and the sensor relay opens to actuate an alarm. In this manner the level of the liquid in a tank may be detected.
In one prior art publication, namely, U.S. Pat. No. 4,717,045 to Peter J. Coppola, a method for detecting the level of sugar syrup is described in which a conductivity probe is suspended above the syrup in a housing with the tip of the probe extending downwardly in contact with the syrup. A sensor is connected to the probe and housing so that a complete circuit is made with the probe, syrup, housing wall, and sensor. When the syrup level drops, the circuit is broken and the sensor reacts to increase the fill level. In this position, syrup on the probe drips quickly and freely so there is no syrup clinging to the probe to produce a false signal after the syrup level has dropped. However, if the syrup level were to rise and completely submerge the probe and adjacent housing wall, a residual film of syrup could act as a conductive path even after the syrup level has dropped below the tip of the probe thereby defeating the purpose of the syrup level detector. Accordingly, it is one object of the present invention to provide a probe and detector arrangement whereby a conductive film or residue will not be present to prevent proper liquid level detection.
It has also been discovered in other probes for use with conductive liquid where the insulation surrounding the probe comes in contact with the conductive liquid that is not sticky like syrup, that a residual conductive film will often be left on the insulation thereby causing bridging that produces false signals. Accordingly, it is another object of the present invention to provide a liquid level detector that avoids the formation of a conductive, bridging film.
The foregoing and other objects are achieved by the present invention which is described below.